Modern computers process their information in a strictly classical manner: that is the evolution of the information in the computer is effectively described by the laws of classical physics. A quantum computer is a device in which the information is made to obey the laws of quantum physics. This shift in how the information in these devices behaves creates a similar shift in the power of these machines. Quantum computers can, for example, efficiently factor numbers—a task that is believed to not be tractable on a classical computer. The promise of quantum computers is that they can exploit quantum effects like interference, superposition, and quantum entanglement to offer computational advantage over classical computers. Large quantum computers, however, are difficult to build in large part because quantum systems like to become classical through the process known as decoherence.
The problem of decoherence is ubiquitous in quantum computer designs. Theoretically decoherence does not pose a fundamental barrier. A famous theorem of quantum computing theory, the quantum threshold theorem, asserts that if the decoherence is slow enough and enough control is maintained over a quantum system, then one can build a large scale quantum computer by using a form of redundancy known as quantum error correction. The requirements for this theorem are, however, daunting. Of particular import are the errors created by the controllers of the quantum systems. These are often the largest errors in the system, and are largely responsible for quantum information losing its coherence. Most of these errors arise from the difficulty of precisely manipulating control fields that interact with the device. Precise laser pulses and voltage waveforms, for example, accompany the vast majority of quantum computer designs.
In order to overcome the precise timing and control issues of quantum computing, a variety of models of quantum computing have been developed. One of the most promising is known as adiabatic quantum computing. In adiabatic quantum computing one engineers a many-body quantum system in such a way as to drag an initial, easily prepared, ground state of a quantum device to a final ground state that holds the result of a quantum computation. It has been shown that this model is equivalent in power to the standard quantum circuit model of quantum computing. These models have the important property of not suffering from timing errors: one need only apply the quantum computation on a slow enough time-scale and not worry about the actual fluctuations in control fields during this evolution. However, constructions for this equivalence all suffer from considerable problems. One major problem is that these constructions are not modular. In particular, without measuring the quantum system devices constructed in an adiabatic quantum computer the constructions cannot be strung together to operate one after the other. A second problem is that none of these models can be shown to be fault-tolerant: there is no known way to make an adiabatic quantum computer tolerant to all the processes of decoherence and error. Adiabatic quantum computers thus offer great potential for quantum computer construction, if the problems of modularity and fault-tolerance can be successfully overcome.